The invention is based on a triggering method for a thyristor switch.
In its more particular aspects the present invention specifically relates to a new and improved triggering method for a thyristor switch containing a multiple number of thyristors encompassing a predetermined number of series-connected thyristors and a predetermined number of thyristors connected antiparallel with respect to the aforementioned series connected thyristors. Such thyristor switch is either triggered or cut-off by means of a predetermined on-off signal for switching-on or switching-off the thyristor switch. Each one of the aforementioned thyristors are triggered or switched-on at least once and when the anode-cathode voltage if such thyristor exceeds a predetermined threshold value.
A triggering method for a thyristor switch is known for example through K. Engberg, H. Frank and B. Klerfors in "Thyristor switched capacitors, TSC, in theory and practise", IEEE Symposium, September 1985, London, pages 183-188. An AC switch employing antiparallel thyristors is described therein which is used for connecting a reactive current compensator capacitor in series with a choke coil to alternating-current conductors. The thyristor which is also designated as a "valve" in such circuits, should expediently switch on if thyristor or valve voltage passes through zero, or, if the thyristor or valve voltage has no passage through zero, at minimum thyristor or valve voltage. In practical operation, the thyristor or valve voltage may have several passages through zero during one half cycle of the alternating current as a result of disturbances such as, for example, those due to current harmonics. In order to ensure even in such cases that the alternating-current switch remains switched on after a switch-on instruction, the thyristors have to be triggered by repeated short-duration trigger pulses or by a long-duration trigger pulse of adequate energy. In the case of reactive power compensation in high-voltage conductors, several thyristors are connected in series, the information for generating the trigger pulses being transmitted from ground potential optionally via glass fibers to the high-voltage potential of the thyristors. The thyristors can then be triggered electrically or directly by optical means. Triggering with repeated short-duration trigger pulses only takes place if the anode-cathode voltage of the thyristor reaches a certain positive value. With thyristors wired in series, said minimum anode-cathode voltage may be reached at different times so that the start of the trigger pulses may vary correspondingly for the individual thyristors. This variation is due to the different capacitance of the individual thyristors (range of variation about .+-.5%).